The invention relates to a measuring procedure and apparatus of a leakage current using a differential detection transformer, and a generator injecting a reference AC signal.
A state-of-the-art measuring procedure consists in:
continuously injecting a reference AC signal between the power system and ground,
taking a measurement signal Vd proportional to the leakage current Id flowing in the leakage impedance Zd at the terminals of a measurement winding of a differential transformer with a toroid around the power system conductors,
analyzing the measurement signal in phase and amplitude by synchronous detection to discriminate between the resistive and capacitive components,
computing and displaying the individual values of the leakage resistance R and of the stray capacitance C of the power system responsible for the leakage current Id. A procedure of this kind is described in the document FR-A 2,616,228.
Monitoring of the isolation of an electrical power distribution system is achieved by analyzing the leakage current Id flowing in the leakage impedance Zd, which is composed of a leakage resistor R connected in parallel to a stray capacitor C (see FIG. 1 of the state of the art). The leakage current Id is created by a generator G which continuously injects into the power system 10, via a filter, a reference AC signal, having a voltage U whose frequency is lower than that of the power system 10. The injection frequency chosen is lower than the power system frequency in order to limit the influence of the stray capacitance C. The leakage resistance R corresponds to the actual isolation fault, and the capacitance C corresponds to the stray capacitances of the lines of the power system 10. The capacitive impedance 1/C .omega. is generally much higher than the resistance R which it is proposed to measure to deduce the isolation level. To isolate the resistive component Ir, the phase difference between the leakage current Id and the injected voltage U of the reference signal has to be able to be measured very accurately. The precision must be in the order of a minute of an angle.
According to FIG. 1, the leakage current Id is measured by means of a differential transformer with a zero sequence toroid 12 disposed around the conductors of the power system 10. The measurement signal Vd generated at the terminals of a load resistor r connected in parallel to the measurement winding 14, is applied to an amplifier 16, and then analyzed by a synchronous detector device 18 which is controlled by control signals C1 and C2 respectively in phase and in phase quadrature with the injected voltage U. Operation of the synchronous detector device 18 is well-known to those specialized in the art and is apparent from the above-mentioned document FR-A 2,616,228.
The output of the synchronous detector device 18 delivers two voltage signals UR and UC respectively proportional to the leakage resistance R, and to the capacitive impedance 1/C .omega.. A computing and display device 20 then indicates the values of the leakage resistance R and of the stray capacitance C. These values would be true if the toroid differential transformer 12 was perfect. State-of-the-art differential transformer technology enables a fairly good precision to be obtained on the amplitude of the signal delivered by the measurement winding 14. The precision on the phase measurement of the leakage signal is, on the other hand, not so good. This difference in precision can be explained by the fact that the error committed on the leakage current amplitude depends on the magnetic permeability of the material of the toroid 12 to the second order only, whereas the error on the phase measurement is a first order function of the same permeability which varies with temperature. This results in the toroid differential transformer 12 introducing an additional phase difference "A" between the leakage current Id and the secondary voltage of the measurement signal Vd of the measurement winding 14. This phase difference A makes the values of R and C displayed by the device 20 in FIG. 1 false. Apart from the influence of the temperature on the permeability, it has moreover been noted that the phase difference A depends on the intensity of the leakage current Id. These different parameters cause a fluctuation of the phase difference A over time which affects the measurement precision. This phase difference fluctuation is unpredictable, and therefore very difficult to model.
It has already been proposed to improve the measurement precision by connecting a capacitor in parallel on the secondary winding 14 of the toroid 12 to form an oscillating circuit tuned to the frequency of the injected voltage U. The phase difference measurement precision is then in the order of five minutes of an angle, which is not compatible with the very high phase difference precision required with a synchronous detector device.
According to another state-of-the-art technique, all the parameters of the toroid 12 are stored in a memory of an isolation monitor equipped with a digital processor. The complexity of the software sequences implemented to take these parameters into account individually in computing the leakage resistance R and the capacitance C makes the processing time of the program longer, and does not however enable the necessary performances to be achieved.
A first object of the invention consists in achieving a simple phase difference correction procedure to improve the measurement precision of a leakage current detected by a differential transformer.